Keyless vehicle entry and start-up systems such as, for example, the Passive Start Entry (PASE) system, are automatic systems for the unlocking of a vehicle without the active use of a vehicle key, and the start-up thereof by the simple actuation of a start button. This is made possible by an electronic key with a chip, which the driver carries with them. Periodically, via at least one antenna which is fitted thereto, the vehicle transmits an encoded request signal in the LF frequency band (LF stands for “Low Frequency”, for example frequencies ranging from 20 kHz to 200 kHz). The system then switches over to a reception mode in the UHF frequency band (UHF stands for “Ultra High Frequency”, for example frequencies in the three-figure MHz range) and awaits confirmation.
If a key fitted with a transponder is within range, the latter receives the LF signal, decodes it, and retransmits it as a newly-encoded UHF signal. The UHF signal is decoded in the vehicle. As the vehicle is familiar with both coding tables, it is able to compare its own original transmission with the signal received and, where the two coincide, to permit access. If no correct response is received within a specified time period, nothing will happen, and the system switches back to standby mode. The engine start-up process essentially corresponds to the access control process, with the exception that, in this case, the engine start button is to be actuated.
As an antenna for the transmission of the LF signal, an inductive antenna is primarily used, which may be configured, for example, as a ferrite core with a winding (also described as a “magnetic antenna” or “ferrite antenna”). In many cases, the inductor coil of the inductive antenna is operated in combination with a capacitor in an oscillating circuit. By the maintenance of the highest possible quality and accurate frequency control, the energy consumption of an oscillating circuit of this type is generally kept low, so that total current consumption of the access and start-up system is maintained as low as possible. Low current consumption, for example, is desirable in itself on the grounds that, where the vehicle is stationary for a prolonged period, the vehicle battery would otherwise be rapidly discharged. However, the requirement for high quality restricts the data transmission rate, and the accurate tuning associated with high quality involves some degree of complexity. In consequence, conventional arrangements generally represent an unsatisfactory compromise between the data rate, complexity and energy consumption.